Energy filter element for ion implantation systems for the use in the production of wafers

ABSTRACT

A method of doping a wafer includes implanting ions into a wafer by irradiating the wafer with an ion beam using an implantation device. The implantation device includes a filter frame and a filter held by the filter frame, wherein the filter is irradiated by the ion beam passing through the filter to the wafer, and the filter is arranged such that protruding microstructures of the filter face away from the wafer and towards the ion beam.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a Continuation of U.S. patent applicationSer. No. 17/036,966, filed on Sep. 29, 2020, which is a Continuation ofU.S. patent application Ser. No. 16/090,521, filed on Oct. 1, 2018,which is a 371 of International application PCT/EP2017/058018, filedApr. 4, 2017, which claims priority of DE 10 2016 106 119.0, filed Apr.4, 2016. The priority of these applications is hereby claimed and theseapplications are incorporated herein by reference.

FIELD OF THE INVENTION

The invention relates to an implantation arrangement comprising anenergy filter (implantation filter) for ion implantation and its use andto an implantation method.

BACKGROUND OF THE INVENTION

By means of ion implantation, it is possible to achieve the doping orproduction of defect profiles, in any desired material such assemiconductor material (silicon, silicon carbide, gallium nitride) oroptical material (LiNbO₃) with predefined depth profiles in the depthrange of a few nanometers to several 100 micrometers. It is desirable inparticular to produce depth profiles which are characterized by a widerdepth distribution than that of a doping concentration peak or defectconcentration peak obtainable by monoenergetic ion irradiation, or toproduce doping or defect depth profiles which cannot be produced by oneor a few simple monoenergetic implantations.

BRIEF SUMMARY OF THE INVENTION

A method of doping a wafer includes implanting ions into a wafer byirradiating the wafer with an ion beam using an implantation device. Theimplantation device includes a filter frame and a filter held by thefilter frame, wherein the filter is irradiated by the ion beam passingthrough the filter to the wafer, and the filter is arranged such thatprotruding microstructures of the filter face away from the wafer andtowards the ion beam.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the basic principle of an energy filter: The energyof a monoenergetic ion beam is modified upon passage through amicrostructured energy filter component as a function of the point ofentry. The resulting energy distribution of the ions leads to amodification of the depth profile of the implanted substance in asubstrate matrix.

FIG. 2 shows, on the left, a wafer wheel, on which substrates to beimplanted are fixed in position. During processing/implantation, thewheel is tilted by 90° and set in rotation. The ion beam, indicated ingreen, therefore “writes” concentric circles on the wheel. To irradiatethe surface of the entire wafer, the wheel is moved vertically duringprocessing. On the right, FIG. 2 shows a mounted energy filter in thearea of the beam opening.

FIG. 3 shows a schematic diagram of various doping profiles (dopantconcentration as a function of depth in the substrate) for energy filtermicrostructures of different configurations (a side view and a top viewshown in each case):

(a) triangular prism-shaped structures produce a rectangular dopingprofile;

(b) smaller triangular prism-shaped structures produce a doping profilewith a shallower depth distribution;

(c) trapezoidal prism-shaped structures produce a rectangular dopingprofile with a peak at the start of the profile;

(d) pyramid-shaped structures produce a triangular doping profile,increasing in height with increasing depth in the substrate.

FIG. 4 shows a cross section through a filter frame for holding anenergy filter chip.

FIG. 5 shows a top view of a filter frame for holding an energy filterelement with a locking element and a mounted energy filter.

FIG. 6 illustrates a typical installation of a frame for holding anenergy filter element in the beam path of an ion implanter. In theexample, the filter holder is arranged on one side of the chamber wall.In the example, this side is the interior side of the chamber wall,i.e., the side which faces the wafer (not shown) during implantation.The frame, into which the filter chip has been inserted, is pushed intothe filter holder and then covers the opening in the chamber wall,through which the ion beam passes during the implantation.

FIG. 7 shows partial frames (on the left in the figure) and a completeframe (at the far right in the figure), each of which can consist of thesame material as that of the energy filter (e.g., monolithic) and/or ofa different material.

FIG. 8 illustrates the use of one or more bars for the attachment of thefilter frame surrounding the filter or of any other passive scatteringelement.

FIG. 9 illustrates the use of a single or multiple suspension elementsfor the attachment of the filter frame surrounding the filter or of anyother scattering element.

FIG. 10 illustrates the use of magnetic fields for the attachment of thefilter frame surrounding the filter or of any other scattering element.

FIG. 11 illustrates a simple realization of a multifilter. Threedifferently formed filter elements are combined in a filter-holdingframe to form a complete energy filter. The ion beam passes uniformlyover all of the individual filter elements.

In the present example (left), the dopant depth profile shown on theright is thus produced. This profile contains three depth profiles,numbered 1, 2, and 3. Each of these subprofiles results from one of thethree subfilters shown on the left, namely, from the subfilter providedwith the corresponding number.

FIG. 12 illustrates a detailed diagram of the multifilter concept. Onthe left are three filter elements shown by way of example. Fourelements are described as numbered. For a given ion species and primaryenergy, a dopant depth profile results from each filter element. Theweighting, i.e., the resulting concentration, is adjustable by varyingthe dimensions of the surfaces of the individual filter elements. Forthis example, it is assumed that the filter and the substrate have thesame energy-dependent stopping power. Usually, however, this is not thecase.

FIG. 13 shows a summation profile obtained when all of the filterelements described in FIG. 12 are mounted together as a complete filterwith appropriate weighting and are exposed uniformly to an ion beam ofappropriate primary energy.

FIG. 14 shows an exemplary arrangement of individual filter elements ina multifilter. The individual elements F1, F2, F3, etc., shown here arecut with a bevel and are mounted directly on each other.

FIG. 15 shows a filter arrangement installed in an ion implantationsystem. Cooling lines, which are supplied with coolant by an externalcooling device, are integrated into the filter holder, which holds thefilter frame. The cooling lines could also be arranged on the surface ofthe filter holder (not shown).

FIG. 16 shows an energy filter with a large surface area, which is onlypartially irradiated per unit time. Thus the non-irradiated areas cancool by radiative cooling. This embodiment can also be configured as amultifilter, as described above. That is, it can be configured as afilter comprising several different filter elements. In the exampleshown, the frame with the filter oscillates in a direction perpendicularto the direction of the ion beam. The area of the filter covered by theion beam is smaller than the total surface area of the filter, so that,per unit time, only a portion of the filter is exposed to the beam. Thisportion changes continuously because of the oscillating movement.

FIG. 17 shows another configuration of an arrangement of energy filters,which rotate around a central axis. Again, the irradiation per unit timeis only partial, and the elements not being irradiated can thus cool.This embodiment is also configurable as a multifilter.

FIG. 18 illustrates schematically a “shifting of the peak”. Byimplanting ions into the energy filter with the help of a trapezoidalprism-shaped structure, a rectangular profile can be produced in thesubstrate. The initial peak is implanted into the energy filter. Thisimplantation profile has the advantageous property that it beginsdirectly at the surface of the substrate, which is extremely importantfor the application of the energy filter.

FIG. 19 shows how ions are implanted in a PMMA substrate by means of anenergy filter during a static implantation. The ions destroy themolecular structure of the PMMA. A subsequent development processreveals the energy distribution of the ions. Areas of high energydeposition are dissolved away. Areas of lower or no energy deposition byions are not dissolved by the developer solution.

FIG. 20 shows a monitoring system for identifying the filter and formonitoring conformity to the filter specifications (maximum temperature,maximum accumulated ion dose).

FIG. 21 shows a collimator structure, which is attached to a filterholder. The aspect ratio determines the maximum angle α. If theavailable distance to the implantation substrate is not large enough,the collimator can consist of several collimator units with smalleropenings arranged side by side. This can be arranged in a honeycombpattern, for example.

FIG. 22 shows a collimator structure built directly on the filter. Theaspect ratio determines the maximum angle α. Here the filter is placedin the ion beam in a back-to-front configuration. If designedappropriately, the collimator has an advantageous mechanicallystabilizing effect on the filter and, as a result of the increase in thesurface area of the filter chip, improves the radiative cooling.

FIG. 23 shows a collimator structure build directly on the filter. Theaspect ratio determines the maximum angle α. In this example, the filteris placed in the ion beam in a front-to-back configuration.

FIG. 24 shows a collimator structure built directly on the filter. Thecollimator structure can have a lamellar, strip-like, tubular, orhoneycomb structure, depending on the layout of the filter and therequired maximum angular distribution.

FIG. 25 shows a collimator structure built directly on the targetsubstrate. The collimator structure can have a lamellar, strip-like,tubular, or honeycomb structure, depending on the layout of thesubstrate structure and the required maximum angular distribution.

FIG. 26 illustrates doping profiles obtained with the same filter butdifferent collimator structures.

FIG. 27 shows doping profiles obtained by means of a multifilter byimplantation with and without a collimator structure.

FIG. 28 illustrates schematically a “turning-over” of the filter. (A)The filter is used in the regular arrangement, which means that themicrostructures point away from the beam. (B) The filter can be turnedover, which means that the microstructures point toward the beam. Thishas advantageous effects on sputtering effects in the filter.

FIG. 29 illustrates schematically a “tilting” of the filter. If theenergy filter is fabricated of anisotropic materials, there can be achanneling effect. This can be prevented by tilting the energy filter.

FIG. 30 shows schematically various doping profiles (dopantconcentration as a function of depth in the substrate) for energy filtermicrostructures of various forms (a side view and a top view shown ineach case). (A) Triangular prism-shaped structures produce a rectangulardoping profile. (B) Smaller triangular prism-shaped structures produce adoping profile with a shallower depth distribution. (C) Trapezoidalprism-shaped structures produce a rectangular doping profile with a peakat the beginning of the profile. (D) Pyramidal structures produce atriangular doping profile, which rises as it extends into the depth ofthe substrate.

FIG. 31 shows various target profile shapes for the same primary ion andthe same primary energy on the basis of different target materials. Thefilter material is silicon in each case.

FIG. 32 illustrates the change in stopping power as a function of energy[4] (SRIM simulation).

FIG. 33 illustrates the starting material of a simple multilayer filter.Filter materials with suitable stopping power are arranged sequentiallyon top of each other by means of a suitable deposition method.

FIG. 34 illustrates how, with a suitable configuration of the layeredstack of materials with different stopping powers, complex dopant depthprofiles can be realized even with a simple filter geometry (here:strip-like triangles).

FIG. 35 illustrates the general principle of constructing an energyfilter out of materials 1-6, each of the individual filter structureshaving a different geometry.

FIG. 36 illustrates the equilibrium charge states of an ion (black line:Thomas-Fermi estimate; blue line: Monte Carlo simulations; red line:experimental results) as a function of the kinetic energy of the ion onpassing through a thin membrane. Ion: sulfur; membrane: carbon [27].

FIG. 37 illustrates the heating of an energy filter by ion bombardment:6 MeV C ions in an energy filter which is not transparent under theseconditions [2].

FIG. 38 shows an embodiment of a filter arrangement in which the filteris held in the filter frame at a defined (positive) potential versus thefilter holder for the purpose of suppressing secondary electrons.

FIG. 39 illustrates the work functions of a number of elements. [25]Materials Science-Poland, Vol. 24, No. 4, 2006.

FIG. 40 shows an arrangement for an energy filter implantation in whichthe complete irradiation of a static substrate is achieved by means ofan ion deflection system before the filter and with the selection of asuitable distance between the filter and the substrate (typically in therange from a few cm to a few m).

FIG. 41 shows an arrangement for an energy filter implantation in whicha large filter surface—larger than the surface of substrate—is used toachieve the complete irradiation of the substrate. The diameter of theirradiated filter area is larger than the diameter of the substrate.

FIG. 42 illustrates a filter only parts of which are active, withmechanical scan in one direction.

FIG. 43 illustrates a modification of the doping profile in thesubstrate by means of a sacrificial layer in the case of a masked,energy-filtered implantation. In the example shown here, the beginningof the implantation profile is shifted into the sacrificial layer. Thisprinciple can be used analogously for unmasked, energy-filtered ionimplantation.

FIG. 44 illustrates a lateral modification of the doping profile in thesubstrate by means of a sacrificial layer in the case of an unmasked,energy-filtered ion implantation. The lateral depth modification comesabout through the lateral differences in the thickness of thesacrificial layer. The principle can be used analogously for masked,energy-filtered implantations.

FIG. 45 illustrates a coupling of vertical movements in the y directionof the filter and the substrate. By the rotation of the wafer wheel, thewafers are guided behind the substrate in the x direction. The ion beam(not shown) is expanded in the x direction, for example, and, as aresult of the vertical oscillation of the implantation chamber, it scansthe entire surface of the multifilter. The surface consists of activefilter regions and inactive holder regions. The arrangement shown in (A)is an unfavorable arrangement. When one considers the irradiated filtersurface at y1 and y2, three filters are irradiated at y1, whereas nofilter is irradiated at y2. As a result, one obtains a laterallyinhomogeneous stripe pattern on the wafer. The arrangement shown in (B)is a possible example of a better arrangement. Two filters areirradiated at y1 and y2. This is true for all y. As a result, alaterally homogeneous doping over the entire surface of the wafer isachieved.

FIG. 46 shows a wafer wheel with an arrangement of wafers to beirradiated and monitoring structures located between the wafers.

FIG. 47 shows a monitoring mask with an example of an arrangement ofvarious mask structures Ma1-Ma10, which are transparent or partiallytransparent to ion beams.

FIG. 48 shows a cross section through a monitoring mask and a monitoringmaterial.

FIG. 49 shows an example of a concentration depth profile produced bymeans of an energy filter.

FIG. 50 shows an example of a mask structure for monitoring thedepth-dependent dose distribution.

FIG. 51 illustrates the monitoring of the implantation process by meansof monitoring structures.

FIG. 52 illustrates the monitoring of the implantation process by meansof monitoring structures.

FIG. 53 illustrates the monitoring of the maximum projected range.

FIG. 54 illustrates a mask structure.

FIG. 55 illustrates another example of a mask structure.

FIG. 56 illustrates another example of a mask structure.

FIG. 57 illustrates a mask structure for monitoring asymmetric angulardistributions.

FIG. 58 illustrates various arrangements of mask structures fordetecting ion angular distributions in various directions.

FIG. 59 illustrates a skillful adaptation of the transition between twoimplantation profiles A and B, so that the resulting overallconcentration profile will result in the desired profile, e.g., ahomogeneous profile. This can (but does not have to) be advantageous inparticular in the case of layer systems consisting of two layers as inthe figure shown here.

Proposal for a realization with the following sequence of processes:

(1) doping the lower layer (implant B),

(2) growing the upper layer, and

(3) doping the upper layer.

For the configuration of the high-energy tail of implant A, there remainonly limited possibilities. The lower-energy tail of implant B, however,can be influenced in particular by the introduction of a sacrificiallayer as described in “15: Modification of the doping profile in thesubstrate by means of a sacrificial layer”.

Proposal for a realization with the following sequence of processes:

(1) growing the sacrificial layer;

(2) doping the lower layer (implant B);

(3) removing the sacrificial layer;

(4) growing the upper layer;

(5) doping the upper layer.

DETAILED DESCRIPTION

FIG. 1 shows a method known from [7] for producing a depth profile. Inthis case, an ion beam is implanted into a substrate by means of astructured energy filter in a system for ion implantation for thepurpose of wafer processing. The implantation method and the dopantdistribution or defect distribution in the wafer after processing areshown. What is shown in particular is how the energy of a monoenergeticion beam is modified as it passes through a microstructured energyfilter component, depending on the point where it enters. The resultingenergy distribution of the ions leads to a modification of the depthprofile of the implanted substance in the substrate matrix. Alsoillustrated in FIG. 1 is this depth profile, which is rectangular in theexample shown here.

FIG. 2 shows a system for ion implantation. This system comprises animplantation chamber, in which several wafers can be arranged on a waferwheel. The wafer wheel rotates during the implantation, so that theindividual wafers repeatedly pass by a beam opening in which the energyfilter is arranged and through which the ion beam arrives in theimplantation chamber and thus strikes the wafers. The wafer wheel, onwhich the substrates to be subjected to the implantation are mounted,can be seen on the left in FIG. 2 . During processing/implantation, thewheel is tilted 90° and set in rotation. The ions of the ion beam,indicated in green, thus “write” concentric circles on the wheel. Toirradiate the surface of the entire wafer, the wheel is moved verticallyduring processing. An energy filter mounted in the beam opening can beseen on the right in FIG. 2 .

FIG. 3 shows by way of example several layouts or 3-dimensionalstructures of filters to illustrate the principle of how a large numberof different dopant depth profiles can be produced by an appropriatechoice of filter. The individual filter profiles shown in FIG. 3 can becombined with each other to obtain additional filter profiles and thusadditional dopant depth profiles. Cross sections of the energy filtersare shown in each case (on the far left in the figures) as well as topviews of the energy filters and curves showing the change in theachieved dopant concentration versus depth (as a function of depth) inthe wafer. The “depth” of the wafer is the direction perpendicular tothe surface of the wafer into which the ions are implanted. As shown inFIG. 3 , (a) triangular prism-shaped structures bring about arectangular dopant profile; (b) smaller triangular prism-shapedstructures produce a rectangular dopant profile extending to a lesserdepth than in case (a) (the depth of the profile can therefore beadjusted by selecting the size of the structures); (c) trapezoidalprism-shaped structures produce a rectangular doping profile with a peakat the start of the profile; and (d) pyramidal structures produce atriangular doping profile which increases in height with increasingdepth in the substrate.

For many reasons, the energy filters (implantation filters) or energyfilter elements known in the past are not adapted to the achievement ofhigh throughputs, i.e., many wafers per hour. It is desirable inparticular to have high wafer throughputs per hour, ease of handling,ease of production, and the realization of any desired profile shapes.Static or movably mounted filters which are produced monolithically,i.e., from a solid block of material, and which are mounted individuallyin the ion beam are known from the literature [2], [3], [4], [5], [6],[7], [8], [9], [10]. In contrast to silicon, the shape of doped regionsin SiC wafers cannot generally be changed by the outward diffusion ofdopant profiles [2], [4], [5], [6]. The reason for this lies in the verysmall diffusion constants—even at high temperatures—of the commondopants such as Al, B, N, and P. These diffusion constants are manyorders of magnitude below the comparison values for silicon, forexample. For this reason, it has not been possible until now to realizedoped regions economically, especially those with high aspect ratios,i.e., a small ratio of base surface area to depth.

Dopant depth profiles in semiconductor wafers can be produced by in-situdoping during epitactic deposition or by (masked) monoenergetic ionimplantation. In the case of in-situ doping, high levels of imprecisioncan occur. Even in the case of homogeneous dopant profiles, the natureof the process means that significant deviations from the ideal dopingare to be expected on the wafer, i.e., from the middle to the edge. Forgradient depth profiles, this imprecision also extends to the verticaldirection of the doped region, because now the local dopantconcentration depends on a large number of process parameters such as,for example, temperature, local dopant gas concentration, topology,thickness of the Prandtl boundary layer, growth rate, etc. The use ofmonoenergetic ion beams means that many separate implantations must becarried out to obtain dopant profiles with acceptable vertical waviness.This approach can be scaled only to a certain extent, and it veryquickly becomes economically unfeasible. Examples of the inventionrelate to the configuration of an energy filter element for ionimplantation systems which makes it possible to meet the requirements onthe use of an energy filter element in the industrial production ofsemiconductor components, especially components based on SiCsemiconductor material. With respect to the use of energy filterelements, production conditions are defined by the following aspects,for example:

1. Technical Ease of Filter Replacement

In a productive environment, i.e., in a factory, production is carriedout even on ion implanters by industrial workers (“operators”), who inmost cases are not trained engineers.

The energy filter is an extremely fragile microstructured membrane,which is difficult to handle without destroying it. So that this filtertechnology can be used economically, it should be guaranteed that, aftera short period of instruction, even non-professionals (i.e.,non-engineers) are able to replace the filter as if it were a tool whichhas become worn or to exchange it for another one in the implantationsystem.

2. Any Desired Vertical Profile Shapes

Novel semiconductor components such as superjunction components oroptimized diode structures require a nonuniform doping curve. The simpleenergy filters described in [1-6], however, produce only constantprofiles. Complicated filter structures such as those described in Rub[8] are technically very elaborate and difficult to realize according tothe state of the art for production methods. The goal is to realizecomplicated vertical profile shapes by the use of uncomplicated, i.e.,easy-to-produce, filter structures.

3. High-Throughput—Cooling Systems in Combination with Filter Movement

Production conditions mean, for example, that typically more than 20-30wafers with a 6″ diameter at a fluence per wafer of approximately 2×10¹³cm⁻² should be produced per hour on ion implanters (typical terminalvoltage on tandem accelerators >1 MV to 6 MV). So that the requirednumber of wafers can be produced in this situation, ion currents of morethan 1 pμA up to several 10 pμA must be used; or powers of more thanseveral watts, e.g., 6 W/cm² must be deposited on the filter (typicalsurface area 1-2 cm²). This causes the filter to heat up. The problem isto cool the filter by appropriate measures.

4. Simple, Low-Cost Production of the Filter Structures for ObtainingHomogeneous, Uniform Depth Profiles

Filter structures can be produced by anisotropic, wet-chemical etching.In the simplest form, the filter structures consist of suitablydimensioned, long triangular lamellae (e.g., 6 μm high, spaced 8.4 μmapart, length of a few millimeters), which are arranged periodically onthe thinnest possible membrane. The production of triangular lamellaewith sharp points is cost-intensive, because the wet-chemicalanisotropic etching must be adjusted precisely. Sharp points, i.e.,non-trapezoidal lamellae, are costly, because etching rates and etchingtimes must be precisely coordinated with each other to obtain thepointed lamellae. In practice, this leads to a great deal of processcontrol work during etching; and, as a result of the nonuniformprocessing to be expected during etching (etching rates of wet-chemicalprocesses are never perfectly reproducible and are never homogeneousover larger areas) on a chip with many hundreds of lamellae, it leads toloss of yield, i.e., imperfectly structured filter elements. The goal isto realize a simple and low-cost method for energy filter production.

5. High Lateral Homogeneity of the Produced Dopant or Defect Region

The energy filters for ion implantation described in the previouslycited documents [2], [3], [4], [5], [6], [7], [8], [9], [10] have aninternal 3-dimensional structure, which leads to differences in thedistances traveled by the ions as they pass through the filter. Thesedifferences in travel distance—depending on the stopping power of thefilter material—produce a modification of the kinetic energy of thetransmitted ions. A monoenergetic ion beam is therefore converted into abeam consisting of ions with different kinetic energies. The energydistribution is determined by the geometry and materials of the filter;i.e., the filter structure is transferred into the substrate by ionlithography.

6. Monitoring the “End of Life” of the Energy Filter

Because of the nuclear interaction of the ion beams with the filtermaterial and because of the thermal load, a typical service life of thefilter results specifically for each ion implantation process with anenergy filter. For an energy filter of silicon with a support layerthickness of approximately 2 μm, regular prong structures on the orderof 8 μm, and an implantation process at 12 MeV nitrogen with currents ofaround 0.1 pμA, the maximum production quantity is approximately 100wafers (6″).

For the sake of the machine operator and for the security of the filtermanufacturer, the total number of wafers processed with a specificfilter should be monitored.

7. Limitation of the Angular Distribution of the Transmitted Ions

For the production of masked dopant regions, i.e., regions with limitedlateral dimensions, especially in cases of high aspect ratios, theangular spectrum of the transmitted ions must be limited to avoid theimplantation of ions under the masking layer.

8. Minimizing Filter Wear Caused by Sputtering Effects

9. Avoidance of Channeling Effects (Lattice Guidance Effects) Throughthe Arrangement of the Filter Relative to the Ion Beam

10. Realization of Complex Dopant Depth Profiles by Means of SimpleFilter Geometry

11. Electron Suppression During Use of the Filter

It is known that, during the transmission of ions through a solid body,the electrical charge of the ions assumes an equilibrium state.Electrons of the primary beam can be given off to the solid or acceptedfrom it. I.e., the transmitted ions, depending on the properties of thefilter material and the primary energy, have, after passage through thefilter, on average a higher or lower charge state [26]. This can lead toa positive or negative charging of the filter.

At the same time, secondary electrons of high kinetic energy can beproduced by ion bombardment of the front or the back side of the filter.

At high current densities such as those required in industrialproduction, the energy filter will heat up. Because of the thermionicelectron emission (Richardson-Dushman law), thermal electrons areproduced as a function of the temperature and the work function of thefilter material.

The distance (in the high vacuum) of the ion accelerator between thefilter and the substrate is typically only a few centimeters or less.That is, the diffusion of thermal electrons (from thermionic emission)and the action of fast electrons (from the ion bombardment) falsify themeasurement of the ion current at the substrate by means of, forexample, a Faraday cup installed there.

12. Alternative Production Methods by Injection Molding, Casting, orSintering

In publications [2]-[15], the methods of microtechnology are proposedfor the production of the energy filter. It is described in particularthat, for the production of the filters, lithographic methods be used incombination with wet-chemical or dry-chemical etching. For filterproduction, anisotropic wet-chemical etching methods by means ofalkaline etchants (e.g., KOH or TMAH) in silicon are preferred.

In the case of filters produced by the last-mentioned method, thefunctional filter layer is produced from monocrystalline silicon. Itmust therefore always be assumed that, when one is bombarding withhigh-energy ions, channeling effects will in principle influence theeffective energy loss in the filter layer in a difficult-to-controlmanner.

13. Arrangement for Irradiating a Static Substrate

An irradiation arrangement is to be used which makes it possible toirradiate a static substance under energy filtration with high lateralhomogeneity over the entire surface of the substrate. Reason: Endstations of irradiation systems frequently do not have the fullmechanical ability to scan the entire wafer (wafer wheel) with apunctiform or nearly punctiform beam spot. On the contrary, many systemshave an electrostatically expanded beam (=stripe in the x direction),which, for example, scans the wafer electrostatically (y direction). Insome cases, partially mechanical scanners are used. I.e., the beam isexpanded in the x direction, and the wafer is (slowly) movedmechanically in the y direction.

14. Arrangement for Taking Advantage of a Large Filter Surface

An irradiation arrangement must be used which makes it possible toirradiate a static or movable substrate under energy filtration withhigh lateral homogeneity over the entire surface of the substrate and todo so over a large filter surface. This makes it possible to lessenthermal effects and degradation effects in the filter.

15. Modification of the Doping Profile in the Substrate by Means of aSacrificial Layer

The energy filter is a tool for manipulating the doping profile in thesubstrate. For certain requirements, it desirable for the doping profilewhich can be produced in the substrate to be manipulated AFTER theenergy filter. i.e., at a location downstream from the energy filter. Inparticular, it is desirable to “push” the near-surface beginning of thedoping profile away from the substrate. This can be advantageousespecially when the beginning of the dopant profile in the substratecannot be adjusted correctly by the filter, for various reasons(especially a loss of ions through scattering). Such a doping profilemanipulation after the energy filter can be achieved by implantationinto a sacrificial layer on the substrate.

16. Lateral Modification of the Doping Profile in the Substrate by Meansof a Sacrificial Layer

For certain applications it is desirable for the doping profile in thesubstrate to show lateral variations. In particular, changes in theimplantation depth of a homogeneous doping profile could be usedadvantageously for edge terminations in semiconductor components. Suchlateral adjustment of the doping profile can be achieved by providing,on the substrate, a sacrificial layer with lateral thickness variations.

17. Adaptation of a Profile Transition Between Several ImplantationProfiles

For certain applications, it is desirable to join two or more profilesto each other along a “seam” at a certain depth, because otherwise therewill be insulation between the layers. This problem occurs in a layersystem especially when the lower end of an upper doping profile or theupper end of the doping profile underneath comprises a slowly taperingconcentration “tail”.

18. Special Arrangement of the Multifilter Concept for CoupledOscillating Movements

If the multifilter is attached to the part of a movable substratechamber which can move linearly back and forth in front of the beam(e.g., in the case of a rotating wafer disk, with a vertical scandirection), the multifilter can be easily moved relative to the beamsimply by moving the substrate chamber. By means of a magnetic or staticscanning device which scans in one direction in front of the filter, usecan be made of a very large multifilter surface, which is equal, forexample, to the product of the vertical oscillation distance times thehorizontal scan distance. The movements of the wafer and of the filterare coupled in this arrangement, which can lead to problems with respectto the lateral homogeneity of the doping. Through the rotation of thewafer wheel, the ion beam “writes” lines on the wafer. As a consequenceof the above-mentioned arrangement, the position of a horizontalirradiated line on the wafer, for example, is coupled to a certainvertical position on the multifilter. A gap between individual filterelements would, for example, result in an inhomogeneously doped line onthe wafer. An arrangement of the filter components in the multifiltermust therefore be selected so that the lateral homogeneity is ensured inspite of the coupling of the linear movements of filter and substrate.

Examples of energy filters, of implantation devices, or of parts ofimplantation devices which meet the previously mentioned productionconditions are explained in the following. It should be mentioned thatthe measures and concepts explained below can be combined with eachother in any way desired, but each of them can also be appliedindividually, in itself.

Point 1: Technical Ease of Filter Replacement

It is proposed that a frame, which makes it easy to handle theimplantation filter, be installed on the filter in question, which isreferred to in the following as the “filter chip”. As shown in FIGS. 4-6, this frame can be configured in such a way that it can be used on theion implantation system in a previously installed frame holder of theproper size. The frame protects the energy filter, allows ease ofhandling, and takes care of electrical and thermal dissipation and/orelectrical insulation (see FIG. 36 ). The frame can be provided with thefilter chip by the manufacturer of the filter elements in a dust-freeenvironment and delivered in dust-free packaging to the ion implantationsystem.

FIGS. 4 and 5 show an example of the geometric and mechanicalconfiguration of a filter frame. The filter held by it can have anydesired surface structure, which is selected in accordance with thedesired doping profile to be obtained with it. The filter holder and/orthe filter frame can be provided with a coating which prevents materialfrom being abraded from the filter frame and filter holder.

The filter frame and filter holder can be made out of metal, preferablyhigh-grade steel or the like. During the implantation process,sputtering effects in the local environment of the energy filter causedby scattered ions must be expected, That is, it must be expected thatmaterial at and near the surface of the frame and filter will be carriedaway. Metal contaminations on the substrate wafer could be theundesirable consequence. The coating prevents such contamination,wherein the coating consists of a non-contaminating material. Whatmaterials are non-contaminating depends on the properties of the targetsubstrate being used. Examples of suitable materials comprise siliconand silicon carbide.

FIG. 4 shows a cross section through a filter frame for holding anenergy filter chip. The energy filter chip, which is also shown in FIG.4 , can be attached to the frame in various ways, such as by the use ofan adhesive or by a mechanical spring. FIG. 5 shows a top view of afilter frame for holding an energy filter element, which is also shownin FIG. 5 . The filter frame comprises a locking element, by means ofwhich the frame can be opened and closed to allow replacement of thefilter. FIG. 6 illustrates an example of the installation of a frame forholding an energy filter element in the beam path of an ion implanter.What is shown in the upper part of FIG. 6 is a cross section through thechamber wall and the filter holder arranged therein. In the example, thefilter holder is arranged on the inside surface of the chamber wall,i.e., the side which is facing the wafer (not shown) during theimplantation process. The ion beam which, during the implantationprocess, passes through the opening in the chamber wall and through thefilter arranged in front of the opening, is also indicated schematicallyin FIG. 6 . The frame with the filter chip inserted into the filterholder covers the opening in the chamber wall through which the ion beampasses during the implantation process. This is shown in the lower partof FIG. 6 , which shows a front view of the chamber wall with the filterholder attached to it.

The frame can consist of the same material as the filter. In this case,the frame can be produced monolithically together with filter and canthus be called a “monolithic frame”. As explained above, the frame canalso be made of a material different from that of the filter, such as ametal. In this case, the filter can be inserted into the frame.According to another example, the frame comprises a monolithic frame andat least one additional frame of a material different from that of thefilter, which is attached to the monolithic frame. This additional frameis, for example, a metal frame.

The frame can completely surround the filter, as explained and shownabove, and as shown on the right in FIG. 7 . According to additionalexamples, the frame does not form the boundary on all (four) sides(edges) of the filter but rather forms the boundary on only three, two(opposite), or only one of the edges of the filter. The term “frame” istherefore to be understood in connection with this description as asolid frame, which completely surrounds the filter on all sides (edge),and also as a partial frame, which extends only around some of the sidesof the filter. Examples of such partial frames are also shown in FIG. 7. FIG. 7 therefore shows various partial frames (on the left in thefigure) and a complete frame (on the far right in the figure). Each ofthese frames can consist of the same material (e.g., monolithic) as theenergy filter or of a different material.

The energy filter or any other scattering element can be mounted in thebeam path of the implanter in various ways by its frame, which can berealized according to one of the above-explained examples. Theabove-explained insertion of the frame into a filter holder is only oneof several possibilities. Additional possibilities are explained below.

According to one example, illustrated in FIG. 8 , the frame can bemounted on the chamber wall by means of at least one bar. In this case,the at least one bar serves as the filter holder. What are shown in FIG.8 are examples of mountings by means of only one bar, by means of twobars, and by means of three bars. It is obvious that more than threebars can also be provided.

According to another example, shown in FIG. 9 , the frame can also beattached to the chamber wall by suspension brackets or suspensionelements. These suspension elements are flexible, for example, and canbe installed between the frame and chamber wall in such a way that theframe is held firmly in position. The suspension elements act in thisexample as the filter holder. FIG. 9 shows examples of attachments withonly one suspension element, with two suspension elements, and withthree suspension elements. Of course, more than three suspensionelements can also be provided.

According to another example, shown in FIG. 10 , the frame with thefilter is held by magnets in a floating (contactless) manner. For thispurpose, the magnets are attached to a front side and a back side of theframe and to the chamber wall in such a way that, in each case, a magneton the chamber wall or on a holder attached to the chamber wall isopposite a magnet on the frame, wherein the facing poles of opposingmagnets are of opposite polarity. As a result of the magnetic forces,the frame floats between the magnets attached to the chamber wall andthe magnets attached to the holder. The magnets on the frame of thefilter can, for example, be realized by thermal vapor deposition or anyother layer-forming method.

Point 2. Any Desired Vertical Profile Shapes

In principle, the geometric configuration of an energy filter for ionimplantation systems can make it possible to realize any desired dopingprofile in a semiconductor material. For complex profiles, this meansthat it is necessary to produce geometrically very demanding3-dimensional etched structures of different sizes and possibly ofdifferent heights such as pyramids, pits with a defined wall slope,inverse pyramids, etc., on the same filter chip.

It is proposed that rectangular profile shapes such as those which canbe produced by simple triangular structures (multifilter) be used toapproximate any desired profile. Under certain conditions, it is alsopossible to use non-triangular structures (e.g., pyramids) as basicelements for the approximation.

That is, it is proposed that a desired doping profile be broken downinto box profiles, for example, and that a triangular filter structurebe produced for each box profile. Then the individual filter chips aremounted in, for example, the frame in such a way that the area weightingcorresponds to the dopant concentration appropriate to the box profileelement in question.

The breakdown of a dopant depth profile is not limited to the triangularstructures shown here. On the contrary, it can comprise additionalstructures, which in the most general case contain ramps or evenconvexly or concavely rising flanks. The flanks do not necessarily haveto rise monotonically, but they can also contain valleys anddepressions. Binary structures with flank angles of 90° are alsoconceivable.

In one example, the filter elements are cut with a bevel and arrangeddirectly next to each other. The beveled cut has the advantage thatthere is no need for an adhesive bond between the filters to block offions at the filter edge. In addition, it is possible in this way to makeoptimal use of the irradiated surface. For the same total filterdimensions and a given ion current, this has the effect of increasingwafer throughput.

FIG. 11 illustrates an example of a simple realization of a multifilter.In the example, three differently shaped filter elements are combined ina frame of the filter holder to form a complete energy filter. At thetop left in FIG. 11 is a cross section through the filter holder withthe three filter elements, and at the bottom left in FIG. 11 is a topview of the filter holder with the three filter elements. On the rightin FIG. 11 is a filter profile which can be obtained with the combinedfilter. When this filter is used as an implantation filter, the ion beampasses over all of the individual filter elements uniformly, so that thedopant depth profile shown on the right in FIG. 11 is produced. Thisprofile contains three depth profiles, numbered 1, 2, 3. Each of thesedepth profiles results from one of the three subfilters shown on theleft, namely, from the subfilter identified by the same number.

FIG. 12 illustrates by way of example the way in which three differentfilter elements, which can be combined into a multifilter, function. Thefigure shows cross sections through each of the individual filterelements, examples of dimensions for these filter elements, and dopingprofiles such as those which can be obtained by means of the individualfilter elements. In FIG. 12 , only the dimensions are given for a fourthfilter element (not shown) by way of example. The weighting, i.e., theresulting concentration or the doping profile, can be adjusted byvarying the dimensions of the surface areas of the individual filterelements. For this example, it is assumed that the filter and thesubstrate have the same energy-dependent stopping power, although thisdoes not necessarily have to be the case. FIG. 13 shows an example of adoping profile which can be obtained when the four filter elementsexplained on the basis of FIG. 12 are combined into a multifilter andused for implantation. This summation profile results from the additionof the filter elements weighted over the surface in question. It isassumed for the diagram that the filter elements described on the basisof FIG. 12 are assembled with appropriate weighting to form a completefilter and are uniformly exposed to an ion beam of appropriate primaryenergy, so that the summation profile shown is obtained.

In the example explained on the basis of FIG. 11 , the individual filterelements of the multifilter are separated from each other by webs of theframe. According to another example, shown in FIG. 14 , the individualfilter elements can also be directly adjacent to each other. FIG. 14shows a cross section through a multifilter comprising several adjacentfilter elements F1, F2, F3. The multifilter is inserted into a filterframe. The individual filter elements F1, F2, F3 are cut with a bevel inthis example and arranged directly next to each other.

Point 3. High-Throughput—Cooling Systems in Combination With FilterMovement

A high wafer throughput for given target dopings can be realized only bymeans of high ion currents. Because between about 20% to about 99% ofthe primary energy of the ion beam is deposited in the filter membrane,i.e., the part of the implantation filter through which the beam passes,the use of a cooling method is proposed as a way of preventing anexcessive rise in the temperature of the filter even at high ioncurrents.

Cooling of this type can be done by means of, for example, one or moreof the following measures a. to c. explained below.

a. Coolant Flow in the Filter Holder

By this means, the heated filter chip is cooled by the dissipation ofheat. FIG. 15 shows an example of a cooled filter holder of this type.What is shown in particular is a cross section through a filter holder,which is attached to a chamber wall of an implanter. In this example,coolant lines are integrated into the filter holder, which accepts thefilter frame. Liquid coolant is supplied to these coolant lines by anexternal cooling device (not shown). Alternatively or in addition, thecoolant lines can also be arranged on the surface of the filter holder(not shown).

b. Movement of the Filter or Ion Beam

It is proposed that, when a rotating wafer wheel loaded with 10-15wafers, for example, is used, the filter or the filter holder beconfigured in such a way that it rotates or oscillates with a linearmovement. Alternatively, the ion beam can be moved electrostaticallyover the filter while the filter remains stationary.

In these variants, the filter is irradiated only partially by the ionbeam per unit time. As a result, the part of the filter not beingirradiated at a particular moment can cool by radiative cooling. Thus itis possible to realize higher average current densities for a givenfilter under continuous use conditions. Examples of how this can berealized are shown in FIGS. 16 and 17 .

FIG. 16 illustrates an energy filter with a relative large surface area,which is only partially irradiated per unit time. Thus the unirradiatedregions can cool by radiative cooling. This embodiment is alsoconfigurable as a multifilter as described above, i.e. as a filter whichcomprises several different filter elements. In the example shown, theframe with the filter oscillates in a direction perpendicular to thedirection of the ion beam, which is indicated schematically. The area ofthe filter covered by the ion beam is smaller than the total area of thefilter, so that, per unit time, only a portion of the filter isirradiated. This part changes continuously as a result of theoscillating movement.

FIG. 17 shows an example of a filter arrangement with several filterelements, which are supported by a rotating filter holder. Each of theindividual filter elements can be structured in the same way, but theycan also have different structures in order to obtain a multifilter. Asshown in FIG. 17 , the individual filter elements traverse a circularpath around the rotational axis (central axis) of the holder as theholder rotates. In this example as well, only partial irradiation occursper unit time; i.e., not all of the filter elements are irradiatedsimultaneously, so that the unirradiated filter elements can cool.

Point 4. Simplified Filter Design

The production of filter elements with prong structures of an exactheight and with perfect sharp points is demanding in terms of processingtechnology and correspondingly expensive.

For simple doping curves (e.g., homogeneous doping), which start fromthe surface of the substrate and require only a simple prong structure,a simplified design and, in association with that, a simplifiedproduction process are to be proposed here.

It is proposed that the microstructured membrane (e.g., prong structure)of the filter be configured with a plateau on the prongs instead ofsharp points, and that the thickness of the support layer of themembrane be dimensioned in such a way that the low-energy dopant peakbeing formed is pushed into the support layer of the filter and thus isnot implanted in the substrate. An example of a filter of this type isshown in FIG. 18 . A cross section (on the left in the figure), a topview (middle), and an example of a doping profile obtainable by the useof the illustrated filter are shown. As illustrated, a rectangularprofile can be produced in the substrate by implantation of ions intothe energy filter through the use of a trapezoidal prism-shapedstructure. The peak at the beginning is implanted in the energy filter;i.e., there is no peak of the dopant profile present within thesubstrate. This implantation profile has the advantageous property thatit begins directly at the surface of the substrate, which can be ofcrucial importance for the application of the energy filter.

As can be seen from the cross section of the filter in FIG. 18 , thisfilter structure comprises plateaus instead of sharp points. Theindividual structural elements are therefore trapezoidal in crosssection. This greatly simplifies the process technology required torealize the filter. It is known that triangular structures can beproduced in silicon by means of, for example, wet-chemical etching withKOH or TMAH. For this purpose, it is necessary for the tips of thetriangles to be masked lithographically. If perfect tips are to beproduced, this leads to the problem that the etching can penetrateunderneath the lacquer or hard mask structure. Without the idea proposedhere, this problem can be solved only by perfect (and thus complicated,cost-intensive) processing. The idea proposed here thus simplifies theproduction of the filter structures quite significantly. The same istrue analogously for modern plasma-supported etching methods such asRIBE (Reactive Ion Beam Etching) or CAIBE (Chemically Assisted Ion BeamEtching).

Point 5. High Lateral Homogeneity of the Produced Doped or Defect Region

The aspect of lateral homogeneity can be significant in staticimplantation situations. When a rotating wafer disk (wafer wheel) isused with, for example, eleven wafers and a stationary ion beam, thehomogeneity is determined by the rotational and translational movementof the wafer disk relative to the ion beam.

Filter-substrate distance: The angular distribution of the transmittedions is energy-dependent. If the filter and the energy of the ions arecoordinated with each other in such a way that, inter alia, verylow-energy ions (nuclear stopping regime) leave the filter, then thereis a wide angular distribution, because large-angle scattering eventsoccur. If the filter and the energy of the ions are coordinated in sucha way that only high-energy ions (only in the electronic regime,dE/dx_(electron)>dE/dx_(nuclear)) leave the filter, the angulardistribution is very narrow.

A minimum distance is characterized in that the structure of the filteris not transferred to the substrate. I.e., for example, for a givendistribution of the scattering angles of the transmitted ions, theseions pass over a lateral distance at least comparable to the period ofthe lattice constant of the ion filter.

A maximum distance is determined by the loss of scattered ions which canstill be tolerated by the application for a given distribution of thescattering angles, especially at the edge of the semiconductor wafer.

FIG. 19 shows the result of an experiment in which ions were implantedthrough an energy filter during a static implantation into a PMMA(polymethyl acrylate) substrate. The ions destroy the molecularstructure of the PMMA, so that a subsequent development process revealsthe energy distribution of the ions by dissolving regions of high energydeposition. Regions of lower energy deposition or without any energydeposition by ions are not dissolved by the developer solution.

The idea proposed here is to produce high lateral doping homogeneity bychoosing the correct the filter-substrate distance both for dynamic andstatic implantation arrangements.

Point 6. Monitoring the “End of Life” of the Energy Filter

Because of nuclear interaction and the pronounced temperature changes(heating of the filter typically to several 100° C.), energy filtersdegrade as a function of the accumulated implanted ion dose.

Once a critical ion dose is reached, the chemical composition, density,and geometry of the filter are changed in such a way that the effects onthe target profile to be realized can no longer be ignored. The criticalion dose depends on the filter material being used, the implanted ionspecies, the energy, the geometry, and the allowed range of variation(=specification) of the target profile.

For each filter implantation process with a given energy, ion species,profile, etc., a specification which contains a maximum temperatureduring the implantation and a maximum allowed accumulated ion dose cantherefore be defined. According to one example, it is provided that theuse of the energy filter is monitored in such a way that the filtercannot be used outside the specified tolerances, even when it is notbeing monitored by an engineer. For this purpose it is proposed thateach filter be detected on the basis of an electronically readablesignature as soon as the filter is inserted into the filter holder onthe implanter, and that this signature be read by means of, for example,a control computer. The signature is for this purpose stored in anelectronically readable memory arranged on the filter. A databasestores, for example, the signatures of the filters which can be used ona certain implanter and their properties, such as the process to whichthe filter is adapted (ion species, energy), and what accumulated doseand what maximum temperature are allowed to be reached. By reading outthe signature and comparing it with the information in the database, thecontrol computer can thus determine whether the filter is appropriatefor a planned implantation process.

FIG. 20 shows a monitoring system for identifying filters and formonitoring compliance with the filter specification (maximumtemperature, maximum accumulated ion dose). Once the filter has beenidentified by the built-in sensors (charge integrator and temperaturesensor), the accumulated dose and the temperature of the filter, forexample, are measured continuously. The implantation process isterminated when one of the specified parameters is reached or exceeded,i.e., when the filter, for example, has become too hot or when themaximum allowable dose has been implanted through the filter. That is,when a certain value no longer meets the specification, a signal istransmitted to the control computer, which terminates the implantationprocess.

Point 7. Limitation of the Angular Distribution of the Transmitted Ions

For applications which require blank regions on the target substrate,masking material can be applied to the target substrate.

To avoid a lateral “blurring” of the structures by a filter-induced,overly broad ion distribution, it is proposed that the ion beamtransmitted through the filter be collimated. The collimation can beaccomplished by strip-like, tubular, lattice-like, or hexagonally shapedstructures with high aspect ratios, which are arranged in thetransmitted beam after the energy filter. The aspect ratio of thesestructures defines the allowed maximum angle.

Various examples are illustrated in FIGS. 21-25 . FIG. 21 shows a crosssection through an implanter chamber wall in the area of the beamopening, a filter holder with an inserted filter fastened to the chamberwall, and a collimator, which, in this example, is attached to the sideof the filter holder facing away from the chamber wall. The aspect ratioof the collimator, which is determined by the length and the width ofthe collimator, determines the maximum angle α, relative to thelengthwise direction of the collimator, at which the ion beam can besent into the collimator and still pass through the collimator. Segmentsof the ion beam which are beamed in at relatively large angles strikethe wall of the collimator and therefore do not pass through it. If thedistance available between the filter and substrate into which the ionsare to be implanted is not sufficient for a desired aspect ratio, thecollimator can also consist of several collimator units with smalleropenings arranged next to each other. They can be arranged in, forexample, a honeycomb pattern.

Alternatively, the collimator structure can also be arranged directly onthe filter element. An element of this type can be produced as amonolith or by means of microbonding methods. Two examples of acollimator structure arranged directly on the filter are shown in FIG.22 . A collimator structure of this type arranged directly on the filtercan mechanically stabilize the filter and also have a cooling effect,because the collimator structure can act as a cooling body with asurface area larger than the surface area of the filter. The maximumangle α is defined here, too, by the aspect ratio of the individualcollimator structures arranged on the filter, each of which comprises alength and a width. The collimator structure can be attached to thefilter by the use of an adhesive, for example, by bonding, or by somesimilar method.

In the example shown in FIG. 22 , the collimator structure is arrangedon the structured side of the filter, that is, where the filter haselevations and depressions. The structures are trapezoidal in thisexample. FIG. 23 shows a modification of the arrangement of FIG. 22 . Inthis example, the collimator surface is arranged on the unstructuredside of the filter. In both cases, the collimator structure is arrangeddownstream from the filter with respect to the direction of the ion beam(symbolized in FIGS. 21 and 22 by the arrow), and therefore in such away that the ion beam passes through the collimator structure only afterpassing through the filter.

FIG. 24 shows top views of collimator structures according to variousexamples. In the examples shown, this collimator structure is arrangedon a filter which has a lamellar structure when viewed from above. Theindividual “filter lamellae” can be, for example, triangular ortrapezoidal in cross section, as already explained above. A lamellarstructure of the filter, however, is only one example. Any other type offilter structure as previously explained can also be used. On the leftand in the middle, FIG. 24 shows an example in which the collimatorstructure is configured in strip-like fashion; i.e., it comprisesseveral parallel strips, each of which extends over the entire width ofthe filter. Each pair of adjacent strips forms a collimator, wherein thewidth of this collimator is determined by the distance between theadjacent strips. The length of the collimator is determined by theheight of the individual strips. The “height” of the strips is theirdimension in a direction perpendicular to the plane of the drawing. Thestrips of the collimator structure can be perpendicular to the lamellaeof the filter, as shown on the left in FIG. 24 , or they can be parallelto the lamellae, as shown in the middle. On the right in FIG. 24 is anexample in which the collimator structure appears as a lattice in a topview, as a result of which a plurality of collimators is formed, thegeometry of which is determined by the geometry of the lattice. In theexample shown, the individual collimators are rectangular, in particularsquare, in a top view, so that the collimators are in the form ofrectangular tubes. This is only an example, however; the lattice canalso be realized in such a way that the individual collimators arecircular, elliptical, or hexagonal (honeycomb-like) in a top view orhave any other desired polygonal geometry.

Collimation by a Hard Mask on the Target Substrate

For masked implantations, it is possible, as an alternative or inaddition to a collimator on the filter, to apply masking to the targetwafer to act as a collimator structure. A condition for this masking canbe that the stopping power of the masking must correspond at least tothe average range of the transmitted ion beam in the target substratematerial. So that the required limitation on the angular distributioncan also be achieved by means of the masking, the aspect ratio of themasking can be adapted accordingly. FIG. 25 shows an example of acollimator structure of this type, which is arranged directly on thetarget substrate. This collimator structure can have any one of thepreviously explained geometries; it can therefore be, for example,lamellar, strip-like, tubular, or honeycomb-like—depending on the layoutof the substrate structure and the required maximum angulardistribution. The aspect ratio of this collimator structure is the ratioof the height (h in FIG. 25 ) to the width (b in FIG. 25 ) of the blankareas of the mask on the substrate forming the collimator structure.

It has been found that the collimator structure influences not only thescattering in the lateral direction but also the depth profile. This isshown in FIG. 26 , which illustrates the doping profiles for threedifferent implantation methods, each of which was conducted with thesame filter but with different collimator structures. In the example,each filter has a lamellar structure with a trapezoidal cross section.This is only an example, however. On the left in the figure, animplantation method is shown in which implantation is carried outwithout a collimator structure. The implantation profile thus obtainedbegins at the surface of the substrate.

In the middle and on the right in FIG. 26 , implantation methods areillustrated in which implantations are carried out with collimatorstructures, wherein the aspect ratio of the collimator structure in thecase of the example on the right is higher than in the case of theexample in the middle. As can be seen, the doping profiles obtained bythese implantation methods do not begin at the surface of the substratebut rather a certain distance away from it, wherein, the higher theaspect ratio, the farther the doping profiles are from the surface andthe flatter their ascent. The explanation for this is that the dopantprofile in the near-surface area of the substrate is caused by ionswhich are more strongly stopped in the filter and thus have a lowerenergy. Such ions with low energy are scattered more strongly by thefilter than ions with higher energy, so that these ions of lower energyhave a wider angular distribution than higher-energy ions. Thus thenumber of low-energy ions which can pass through the collimatorstructure is smaller than the number of higher-energy ions, wherein thegreater the aspect ratio of the collimator structure, the morepronounced the effect, i.e., the smaller the allowable maximum angle atwhich the ions can still pass through the collimator structure.

So that, in spite of the collimator structure, a nearly homogeneousdoping profile can be produced starting from the surface, the filter canbe designed so that low-energy ions are “preferred”. That is, morelow-energy ions than higher-energy ions pass through the filter. Anexample of this type of filter is shown in FIG. 27 . In this example,the filter has different filter regions, each of which has a maximum anda minimum thickness. The maximum thickness is the same in all threeregions, but the minimum thickness differs. This is achieved in theexample in that the filter has, in each of the individual regions, atrapezoidal structure arranged on a base, wherein the bases havedifferent heights or thicknesses, and thus the trapezoidal structuresare of different heights. In a first section, the base is the thinnest,and the trapezoidal structure is the highest, as a result of which thedistance CD1 between adjacent structures is the greatest in thissection. In a third section, the base has the greatest thickness, andthe trapezoidal structure is the lowest, as a result of which thedistance CD3 between adjacent structures is the smallest in thissection. In a second section, the thickness of the base is between thethickness of the first section and the thickness of the third section.Correspondingly, the height of the trapezoidal structure in this sectionis between the height of the first section and the height of the thirdsection, and the distance CD2 between adjacent structures is, in thissection, between the distance CD1 in the first section and the distanceCD3 in the third section. The individual sections can be of the samesize in terms of their surface area, but they can also differ in size.It is also obvious that more than three sections with different minimumfilter thicknesses can be provided.

FIG. 27 shows on the left an implantation profile obtained byimplantation with the filter just described when implantation is carriedout without a collimator structure. This implantation profile begins atthe surface, but the doping concentration decreases in a stepwise mannerwith increasing depth. In this figure, CD1 designates a region of thedoping profile which is attributable to the first section of the filter;CD2 designates a region of the doping profile attributable to the secondsection of the filter; and CD3 designates a region of the doping profileattributable to the third section of the filter. It can be seen on thebasis of the doping profile that, the greater the minimum thickness ofthe base of the associated section, the less deeply the ions passingthrough the filter region in question penetrate into the substrate,i.e., the lower their energy. The doping profile also shows that morelow-energy ions pass through this filter than high-energy ions. Because,as explained above, the low-energy ions are scattered more strongly thanthe high-energy ions and thus fewer low-energy ions pass through acollimator structure than higher-energy ions do, a nearly homogeneousdoping profile beginning at the surface can be obtained by the use ofsuch a filter in combination with a collimator structure. This is shownon the right in FIG. 27 , where an implantation method using theexplained filter and a collimator structure is shown. The collimatorstructure in this example is located on the substrate, but it could alsobe arranged on the filter.

Point 8. Less Filter Wear Caused by Sputtering Effects

Implantation arrangement of the filter relative to the substrate: in onecase the prongs face the substrate, in the other case the prongs faceaway from the substrate (→sputtering, scattering on impact). During thepreviously explained implantation methods and those to be explainedbelow, the filter can be used in each case in such a way that themicrostructures of the filter are facing the substrate, i.e., arepointing away from the ion beam, as shown in FIG. 28(a). Alternatively,the filter can also be rotated, so that the microstructures of thefilter are facing away from the substrate, i.e., are facing toward theion beam, as shown in FIG. 28(b). The latter can have an advantageousinfluence on sputtering effects.

Point 9. Avoidance of Channeling Through the Arrangement of the FilterRelative to the Ion Beam

Tilting of the Filter and/or of the Substrate

Insofar as the filter and/or the substrate consists of crystallinematerial, undesirable channeling effects can occur. That is, ions canachieve increased range along certain crystal directions. The magnitudeof the effect and the acceptance angle are functions of temperature andenergy. The implantation angle and the crystallographic surfaceorientation of the starting material used for the filter and thesubstrate play a crucial role. In general, the channeling effect cannotbe reproduced with certainty across a wafer, because the above-mentionedparameters can differ from wafer to wafer and from one implantationsystem to another.

Channeling should therefore be avoided. Tilting the filter and thesubstrate can prevent channeling. Channeling in the filter or in thesubstrate can have quite different effects on the depth profile of theimplanted dopant, especially when the filter and the substrate consistof different materials.

FIG. 29 shows schematically a filter which is tilted in such a way withrespect to the substrate during the implantation process that a basesurface of the filter forms an angle with a surface of the substratewhich is greater than zero. This angle is, for example, greater than 3°,greater than 5°, or greater than 10° and less than 30°. Especially whenthe energy filter is fabricated of anisotropic materials, it is possiblein this way to prevent or reduce a channeling effect.

Point 10. Realization of Complex Dopant Depth Profiles with a SimpleFilter Geometry

As explained above, more complex dopant depth profiles can be realizedby adapting the geometric design of the filter element. For the sake ofsimplicity in the following explanations, scattering effects of alltypes are ignored.

For the case in which the stopping power for ions (dE/dx) is the same inboth the filter and the substrate material, situations such as thoseshown in FIG. 30 by way of example can occur. FIG. 30 shows a schematicdiagram of various doping profiles (dopant concentration as a functionof depth in the substrate) for energy filters of different designs, eachof which is shown in a side and a top view. As illustrated, (a)triangular prism-shaped structures produce a rectangular doping profile;(b) smaller triangular prism-shaped structures produce a lessdepth-distributed doping profile than the larger triangular prism-shapedstructures shown in (a); (c) trapezoidal structures produce arectangular doping profile with a peak at the start of the profile; and(d) pyramidal structures produce a triangular doping profile whichincreases in height with increasing depth in the substrate.

When, for example, silicon is used as the substrate material to be dopedwith boron, for example, and when the energy filter is made of adifferent material, changes in the dopant depth profile in the substrateare obtained depending on the density and the change in dE/dx of thefilter as a function of the actual kinetic ion energy. A perfectlyhomogeneous, i.e., constant, change in the depthwise doping is achievedonly when identical materials are used for both the filter and thesubstrate. This is illustrated in FIG. 31 , in which doping profiles invarious substrate materials (target materials) are shown, obtained byidentical implantation processes, i.e., implantation processes with thesame primary ion and the same primary energy. The filter material wassilicon in each case. The doping profiles differ as a result of thedifferent substrate materials.

FIG. 32 illustrates the change in the stopping power as a function ofenergy [4] (SRIM simulation) for the various substrate materials onwhich the diagram in FIG. 31 is based.

It is now proposed that, for a given surface geometry, theenergy-dependent change in the stopping power be adapted by means of,for example, the design of the filter as a multilayer system.

It is proposed that the change of the stopping power as a function ofion energy (i.e., as a function of the vertical position in a filterprong for a given ion species and primary energy) be modeled in such away that, overall (i.e., from the entry of the ion into the filter untilthe end position in the irradiated substrate), the total loss of kineticenergy is obtained, depending on the entry position on the filter (moreprecisely, on the actual route of the ion through the filter andsubstrate). The energy loss in the filter is therefore no longerdetermined only by the irradiated length of the filter material butrather by the location-dependent change in the stopping power.

Thus, with appropriate modeling and a fixed geometry, for example, it ispossible to produce doping curves which either rise or fall in thedepthwise direction. The stopping power therefore becomes a function ofthe lateral position. Examples of such filters are shown in FIGS. 33-35. The lateral position in each of these figures is designated “y”.

FIG. 33 illustrates a multilayer starting material for a multilayerfilter. In this example, the starting material comprises four differentlayers, designated 1-4. The use of four layers, however, is only anexample. Fewer than four or more than four layers could be used. Theindividual layers can differ not only with respect to the material usedbut also their thickness. It is also possible for two layers to comprisethe same material and be separated by two or more layers of a differentmaterial. The individual layers can be deposited or producedsequentially, one on top of another, by suitable deposition methods.

With a suitable configuration of the layered stack of materials withdifferent stopping powers, complex dopant depth profiles can be realizedeven with a simple filter geometry. FIG. 34 shows a cross sectionthrough a filter which was realized on the basis of the startingmaterial produced in FIG. 33 and which, in the example shown, comprisesa base and triangular structures arranged on the base. These triangularstructures can be in the form of strips, i.e., they can extend in adirection perpendicular to the plane of the drawing, or they can be partof pyramidal structures.

As shown in FIG. 35 , the filter can also be realized in such a way thatseveral structures are arranged next to each other in the lateraldirection (y direction), these structures comprising differentgeometries and/or different layered stacks, i.e., layered stacks with adifferent construction with respect to the sequence of individual layersand/or of the material of the individual layers. For example, sixdifferent materials are used in the filter shown, these layers beingdesignated 1-6.

Silicon, silicon compounds, or metals, for example, are suitable asmaterials for the individual layers, although the choice is not limitedto such materials. Silicon compounds include, for example, siliconcarbide (SiC), silicon dioxide (SiO₂), and silicon nitride (SiN).Suitable metals include, for example, copper, gold, platinum, nickel,and aluminum. According to one example, at least one layer of a siliconcompound is grown on a silicon layer, and a metal layer is depositedfrom the vapor phase on the at least one layer of the silicon compound.A metal layer can also be vapor-deposited directly onto a silicon layer.The possibility also exists of producing various metal layers on top ofeach other by vapor deposition so as to obtain a filter with differentlayers.

Point 11. Electron Suppression

It is known that, during the transmission of ions through a solid,electrons of the primary beam remain in the solid or are taken up by theion. I.e., depending on the properties of the filter material and theprimary energy, the transmitted ions have, after passage through thefilter, on average a higher or lower charge state [26]. Electrons aregiven up to the filter or accepted.

FIG. 36 shows the equilibrium charge states of an ion (black line:Thomas-Fermi estimate, blue line: Monte-Carlo simulations; red line:experimental results) as a function of the kinetic energy of the ion onirradiation of a thin membrane. Ion: sulfur. Membrane: carbon [27].

As a result of ion bombardment, it is possible for secondary electronswith high kinetic energy to be produced on both the front and back sidesof the filter simultaneously. At high current densities such as thoserequired in industrial production, the energy filter will heat up.

As a result of thermionic electron emission (Richardson-Dushman law),thermal electrons are produced as a function of the temperature and workfunction of the filter material. This is shown in FIG. 37 , whichillustrates the heating of an energy filter by ion bombardment. Thecurve shown is based on an experiment in which a filter was irradiatedwith carbon (C) ions with a high energy of 6 MeV. The filter in thiscase was a non-transparent energy filter [2].

The distance (in a high vacuum) of the ion accelerator between thefilter and the substrate is typically only a few centimeters or less.Thus the measurement of the ion current at the substrate by means of,for example, a Faraday cup attached there is falsified by the diffusionof thermal electrons (from thermionic emission) and through the actionof fast electrons (from ion bombardment).

As previously described, there are, from the viewpoint of the filter,both processes which yield electrons (stripping of the primary ions) andprocesses which emit electrons. Thus the potential of a mounted,electrically insulated filter is not well defined. On the contrary, itvaries as a function of the ion current, the vacuum conditions,temperature, etc., during the implantation process. A net-negativecharging will promote the emission of electrons, whereas a net-positivecharging has the tendency to suppress the emission of electrons. Variousways in which such charging can be prevented are explained below.

(a) Energy Filter Element at a Defined (Positive) Potential

It is proposed that the energy filter be designed and mounted in such away that the filter is always at a defined potential during the ionbombardment. FIG. 38 shows a cross section through a filter arrangementin which this is ensured. With this filter arrangement, the filter inthe filter frame is held at a defined (positive) potential versus thefilter holder for the purpose of suppressing secondary electrons. Thefilter frame is connected to a voltage source and is electricallyinsulated from the filter holder and the chamber wall of the implanter.

The electrical potential of the filter holder can be regulated. This canbe done in such a way, for example, that, independently of the chargebalance which results from the implantation process, a constantpotential versus the potential of the substrates to be implanted orversus ground potential is maintained during the implantation. For thispurpose, a controlled feed of positive or negative charge by means of acurrent source can be provided.

The potential to be maintained can be selected in particular in such away that, for example, the emission of electrons from the filter iscompletely suppressed and thus only the (positive) charge of thetransmitted ion current is measured in the Faraday cup next to or on thesubstrate. Typical values for such a (positive) potential are in therange between a few 10 V and a few 1,000 V.

For the case that the energy filter, because of its materialcomposition, is very high-ohmic, it is proposed that the filter beprovided with a thin, highly conductive layer with a thickness rangingfrom a few nanometers to a few tens of nanometers on one or both sides.The stopping power of this layer must be incorporated into the overallbalance of the stopping power when the filter is being designed. Caremust be taken to ensure that the applied layer (even when applied to theside facing away from the substrate) cannot in principle cause anydamaging contamination of the substrate material to be implanted. Forthe processing of SiC substrates, the layer can consist of carbon, forexample.

(b) Energy Filter is Coated with a Material with a High Work Function

To reduce strong thermionic emission, it is proposed that the energyfilter be coated on one or both sides with a material with a highelectron work function, so that, at a given temperature, the leastpossible thermionic emission is caused. The work functions of a fewelements are shown in FIG. 39 [25], Materials Science Poland, Vol. 24,No. 4, 2006. In particular, materials with a work function of greaterthan 3.5 eV, greater than 4 eV, or greater than 4.5 eV are suitable.

The stopping power of this layer should be included in the overallbalance of the stopping power calculated when the filter is designed.Care must be taken to ensure that the applied layer (also when appliedto the side facing away from the substrate) will not in principle causeany harmful contamination of the substrate material to be implanted.

Point 12. Alternative Production Methods Using Injection-Molding,Casting, or Sintering

Implantation filters can be produced by the methods of microtechnologysuch as lithography in combination with wet-chemical or dry-chemicaletching. In particular, anisotropic wet-chemical etching by means ofalkaline etchants (e.g., KOH or TMAH) is used for filter production outof silicon. In filters of this type, the functional filter layer is madeof monocrystalline silicon. During bombardment with high-energy ions,channeling effects can thus influence the effective energy loss in thefilter layer in a difficult-to-control manner. Examples of how sucheffects can be avoided are explained below.

(a) In one example, it is provided that, for production by a typicalmicrotechnology process, wet-chemical anisotropic etching, whichrequires monocrystalline material, is replaced by dry-chemical etching,which means that polycrystalline or amorphous starting material is usedfor the filter membrane. Because of its material structure, theresulting filter shows improved properties with respect to channeling.

(b) In another example, it is provided that the filter is not producedby a typical sequence of microtechnical processes but rather byimprinting, injection-molding, casting, or sintering. The core idea isto implement the above-mentioned processes in such a way that a mold ormold insert determines the final shape of the energy filter membrane.The selected filter material is now processed in the familiar manner forthe method in question. That is, it is given the required geometry in asoft state (for imprinting), a liquid state (for injection-molding andcasting), or in a granular state (sintering) by the predeterminedcasting mold, mold insert, die, etc.

One of the advantages of the use of the above-mentioned methods is thatmonocrystalline filter membranes are typically not obtained, and thuschanneling is suppressed. Another advantage is that the range ofavailable filter membrane materials is very large. For example, the useof filter membranes of sintered SiC for the production of homogeneousdoping profiles in SiC substrates is especially advantageous.

Another advantage is that, through the use of the above-cited moldingmethods, the cost of the production of a large number of filter elementscan be reduced considerably in comparison to the cost of production bymicrotechnology.

Point 13. Irradiation of a Static Substrate

The homogeneous, energy-filtered irradiation of a static substrate canbe achieved by “wobbling” (=controlled deflection) of the ion beambefore the filter, arranging the filter between the wobbling unit (=ionbeam deflection system) and the static substrate; and choosing thecorrect deflection angle and distance d between the filter and thesubstrate (usually a few cm to m), as shown in FIG. 40 by way ofexample. FIG. 40 illustrates an arrangement for an implantation into asubstrate through an energy filter. This arrangement comprises adeflecting setup for the ion beam, which is arranged before the filter.The deflection of the ion beam which can be achieved by this deflectingsetup is coordinated with the distance between the filter and thesubstrate (typically in the range from a few cm to several m) in such away that the substrate can be completely irradiated, i.e., its entiresurface can be irradiated, for the purpose of implantation.

Point 14. Arrangement for Exploiting a Large Filter Surface Area

(a) Arrangement in which the Entire Filter Surface is Active

FIG. 41 shows an arrangement for an energy filter implantation (i.e.,implantation by means of an energy filter), in which the beam area hasbeen enlarged by suitable measures, and the irradiated filter surfacearea is larger than the substrate surface area, as a result of which itis possible to irradiate the substrate completely, and use can be madeof a large filter surface. The diameter of the irradiated filter regionis larger than the diameter of the substrate. The substrate can bestatic or movable. As a result of this arrangement, it becomes possibleto use a large filter surface (=reduction of the degradation effects andthermal effects in the filter), and it is ensured that the entiresurface of the substrate can be irradiated. The use of an arrangement ofthis type is especially advantageous in cases where the required filterstructures are “large”. For the doping of high-blocking Si-IGBTs or Sipower diodes with protons, penetration depths of >100 μm are required.For this application, therefore, filter structures with “prong heights”of >100 μm must be provided. Such filter structures can be produced veryeasily with sufficient mechanical stability, even for large substrates(e.g., 6″ or 8″).

In the arrangement described here, a certain minimum distance should bemaintained between substrate and filter which ensures that there issufficient lateral homogenization of the implanted ions as a result ofscattering effects.

(b) Arrangement in which Part of the Filter Surface is Inactive

The same arrangement as that described in section 14(a) is used first:an arrangement for an energy filter implantation in which the beam areahas been enlarged by suitable measures and the irradiated filter surfaceis larger than the substrate surface. Nevertheless, the entire filtersurface is not active here. On the contrary, only a certain portion ofthe filter surface is active. This means that the filter consists of anarrangement of a number of filter elements in the form of, for example,strips. These filter elements can be produced monolithically from asubstrate, for example, by appropriate production processes. The other(non-active) part of the filter surface is used to stabilize the filtermembranes. This part causes the ion beam to cast a shadow. In thisarrangement, therefore, either the substrate or the filter must be movedin order to compensate for the shadowing effect. As a result of thisarrangement, it becomes possible to make use of a large filter surface(=reduction of the degradation effects and thermal effects in thefilter), and it is ensured that the entire surface of the substrate canbe irradiated. FIG. 42 illustrates a filter with inactive parts and withmechanical scan in one direction.

Point 15. Modification of the Doping Profile in the Substrate by Meansof a Sacrificial Layer

In addition, a sacrificial layer, the thickness and stopping power ofwhich are selected appropriately, can be applied to the substrate, sothat the implantation profile is shifted in the depthwise direction inthe substrate in the desired manner. A sacrificial layer of this typecan be used for masked ion implantation (compare FIG. 43 ) and also forunmasked ion implantations. In particular, this method makes it possibleto “push” an undesirable beginning part of a doping profile away fromthe substrate and into the sacrificial layer, i.e., to implant thebeginning part of the profile in the sacrificial layer.

FIG. 43 illustrates a modification of the doping profile in thesubstrate obtained by means of a sacrificial layer in the case of amasked, energy-filtered implantation. In the example shown here, thebeginning part of the implantation profile is pushed into thesacrificial layer. This principle can be exploited analogously for anunmasked, energy-filtered ion implantation, i.e., for an implantation inwhich no masking layer is present, in contrast to FIG. 43 .

Point 16. Lateral Modification of the Doping Profile in the Substrate byMeans of a Sacrificial Layer

By applying a sacrificial layer, the stopping power of which and thechange in its thickness over the wafer surface are appropriatelyselected, to the substrate, the implantation profile can be shifted asdesired in the depthwise direction into the substrate as a function ofthe lateral position on the wafer. A sacrificial layer of this type canbe used for masked ion implantation and also for unmasked ionimplantations (compare FIG. 44 ). In particular, the change in theimplantation depth of a homogeneous doping profile can be usedadvantageously for edge terminations in semiconductor components.

FIG. 44 shows a lateral modification of the doping profile in thesubstrate by means of a sacrificial layer in the case of an unmasked,energy-filtered ion implantation. A lateral modification of theimplantation depth is achieved here by variation of the thickness of thesacrificial layer in the lateral direction. The principle can be usedanalogously for masked, energy-filtered implantations.

Point 17. Adaptation of a Profile Transition Between SeveralImplantation Profiles

Two or more doping profiles can be skillfully overlapped, so that adesired overall doping profile is obtained, especially in the area ofthe overlap. This technique is advantageous especially for the growingand doping of multiple layers. A representative example consists of thegrowing of several SiC-epi layers and their energy-filtered doping. Goodcontact between the layers must be ensured.

Point 18. Special Arrangement of the Multifilter Concept with CoupledOscillating Movements

A skillful arrangement can be used so that, in spite of coupledoscillating movements of filter and substrate, i.e., no relativevertical movement between filter and substrate, the lateral homogeneityof the distribution of the ions can nevertheless be achieved. Anarrangement of this type is shown in FIG. 45 . The wafers are guided bythe rotation of the wafer wheel in the x direction behind the substrate.The ion beam (not shown) is, for example, expanded in the x directionand scans the entire multifilter surface by the vertical oscillatingmovement of the implantation chamber. The filter surface consists ofactive filter regions and inactive holder regions. Arrangement (A) is anunfavorable arrangement. When one considers the irradiated filtersurface for y1 and y2, three filters are irradiated at y1, and no filterat all is irradiated at y2. As a result, one obtains a laterallyinhomogeneous stripe pattern on the wafer. Arrangement (B) shows apossible example of a better arrangement. Two filters are irradiatedboth for y1 and for y2. This is true for all y. As a result, a laterallyhomogeneous doping over the wafer surface is achieved.

As shown in FIG. 45 , the vertical movements in the y direction offilter and substrate are coupled. The wafers are guided along behind thefilter in the x direction by the rotation of the wafer wheel. The ionbeam (not shown) is, for example, expanded in the x direction and scansthe entire multifilter surface by the vertical oscillating movement ofthe implantation chamber. The filter surface consists of active filterregions and inactive holder regions. Arrangement (A) is a ratherunfavorable arrangement. When one considers the irradiated filtersurface for y1 and y2, three filters are irradiated at y1, and no filterat all is irradiated at y2. As a result, one obtains a laterallyinhomogeneous stripe pattern on the wafer. Arrangement (B) shows apossible example of a better arrangement. Two filters are irradiatedboth for y1 and for y2. This is true for all y. As a result, a laterallyhomogeneous doping over the wafer surface is achieved.

Point 19. Monitoring

Another aspect is intended to solve the problem of monitoring importantparameters of the ion implantation as modified by the energy filter.Such parameters are, for example, the minimum or maximum “projectedrange”, the depth concentration distribution determined by the filtergeometry, and the (energy-dependent) angular distribution. Themonitoring of other parameters such as the implanted ion species, etc.,could also be useful. Monitoring should be possible in particular on thewafer in which the ions are implanted or (simultaneously) on (several)structures which are near the wafers. According to one aspect, themonitoring should be conducted without the need for any furtherprocessing of the monitoring structures or of the wafers.

Monitoring can be conducted by measuring optical parameters such asspectral absorption, spectral transmission, spectral reflection, changesin the index of refraction, global absorption (wavelength rangedependent on the measuring device), and global transmission as well asreflection (wavelength range dependent on the measuring device).

According to one aspect, it is provided that, for the monitoring of theabove-mentioned implantation parameters, arrangements of masks andsubstrate materials be used which (1) are arranged at an appropriatelocation on the surface to be implanted, e.g., the wafer wheel, andwhich (2) change their optical properties, for example, as a result ofthe ion implantation in an “as-implanted” manner, i.e., without the needfor further post-processing, in such a way that, for example, the changeis proportional to the implanted ion dose for a given ion species. Suchmaterials cited under (2) are, for example, PMMA (Plexiglas), PMMA, SiC,LiNbO₃, KTiOPO₄, etc.

In cases where the material which is optically sensitive to ionradiation is simultaneously the material of the target substrate, thetarget substrate (e.g., an SiC wafer) can be used directly for theoptical monitoring.

In addition to changes in the optical properties, it is known thatmaterials such as PMMA change their solubility in certain acids andsolvents after irradiation by ions. Thus the depth (or the etching rateor the resulting etch geometry, etc.) of a structure modified after ionirradiation can be considered a measure of the implanted ion dose.

Monitoring of other changes in physical parameters by ion irradiation isconceivable. Such changes can include, for example, mechanicalproperties of the monitor material, electrical properties of the monitormaterial, or even the nuclear-physical activation of the monitormaterial by high-energy ion irradiation.

According to one aspect, the detection should proceed on the basis ofchanges in the optical properties. An implementation of this type is tobe described in the following. For the frequently occurring case thatmonitoring does not take place on the target substrates to be implanted,the implementation of separate monitoring structures is proposed. Amonitoring structure consists of an arrangement of a suitable substratematerial with one or more mask structures. Examples are shown in FIGS.47 and 48 .

The monitoring structure or structures (monitoring chips) are, as shownin FIG. 46 , arranged at a suitable location, such as on the waferwheel. The read-out of the monitoring chips is done after theimplantation without further post-processing, for example. In certaincases, the mask must be removed from the monitoring substrate for theread-out measurement. According to one aspect, the mask is reusable.

Mask material and substrate material of the monitoring chip can alsoconsist of different materials. A criterion for the selection of themask material is, for example, its compatibility with the material ofthe target substrates (to exclude contamination caused by sputteringeffects). Another criterion is a stopping power for high-energy ionssuch that mask structures with high aspect ratios can be produced.

It is also possible that the mask material and the substrate material ofthe monitoring chip could be produced out of the same material. Mask andsubstrate can also be produced monolithically. In this case, it isusually impossible to reuse either the mask or the substrate.

Performance and evaluation of the masked structure after ionimplantation:

1. performance of the energy filter-modified ion implantation;

2. removal of the mask—possible but not absolutely necessary, becausethe read-out measurement can also be done by reflection from the rearsurface of the substrate;

3. optical measurement:

a. absorption spectrum, wavelength-resolved

b. transmission spectrum, wavelength-resolved

c. reflectivity, wavelength-resolved

d. simple global absorption, i.e., not wavelength-resolved

e. simple global transmission, i.e., not wavelength-resolved

f. measurement of the change in the index of refraction

g. change in polarization

4. comparison with calibration curve or comparison standard and thusdetermination of whether the implant process has taken place asexpected.

Through the use of the explained monitoring structures, the followingimplant parameters can be tested:

A. Depth-Dependent Dose

This is therefore a test for the degradation of the filter and a test todetermine if the implant dose was correctly set on the machine side.

B. Maximum/Minimum Projected Range

This is therefore a test of the correct implant energy, a test fordegradation of the filter and for the correctness of the filterstructures produced.

C. Angular Distribution of the Implanted Ions

This is therefore a test for the degradation of the filter, a test ofthe correct formation of the filter, and a test for the correctgeometric arrangement in the implantation chamber.

A. Monitoring the Depth Distribution of the Implanted Ions

Point A. Depth-Dependent Dose

FIGS. 49-52 show by way of example the monitoring of the depthdistribution of the implanted ion dose set by the energy filter. In thisexample, the following simplifying assumptions apply:

-   -   The change in the optical properties is produced only by the        locally implanted ion dose and the eigen-defects thus caused.    -   Ions which, for example, cross through only depth region III        with ion concentration C1 (FIG. 51 ) (so that they arrive in        concentration region C2) lead to no further change in the        optical properties.    -   It is conceivable that precisely such a change in the optical        properties might be observed in PMMA, for example, as a result        of electronic stopping.    -   This is not a problem for the possibility of evaluation in        principle, but in the example of FIGS. 51 and 52 , it will be        excluded for the sake of simplicity.

The mask structures shown or described in FIG. 50 are staggered withrespect to thickness and number, depending on the desired depthresolution, i.e., configured as a “slanted plane” or continuous ramp.For the greatest thickness, the following therefore applies by way ofexample: “thickness of the mask”>R_(p,max).

The lateral dimensions of the individual structures can range fromsquare micrometers to square millimeters to square centimeters,depending on the requirements of the read-out apparatus.

Point B. Monitoring the Maximum Projected Range

FIG. 53 shows a structure which is adapted to the monitoring of themaximum projected range.

Analogous structures, using evaluation procedures like those describedunder A, can also be used to measure or monitor the minimum projectedrange.

Point C. Monitoring the Angular Distribution of the Implanted Ions

It is known that the energy filter for ion implantation produces anenergy-dependent spectrum of ion angles after passage through thefilter.

In principle, it is true that, for monoenergetic ions arrivingperpendicularly to the surface of the filter, the resulting lower-energyions after the filter are scattered more strongly than the high-energyones.

The resulting angular distribution is therefore a function of the filtergeometry, the change in the geometry during the service life of thefilter, the occurrence of channeling effects, the ion species beingused, the primary energy, the resulting maximum and minimum energy ofthe transmitted ions, and the geometric arrangement in the implantationchamber. It is possible to keep track of all these parameters bymonitoring the angular distribution.

For the monitoring of individual parameters, different mask structures,which can be arranged in a monitoring chip, are proposed, as illustratedand described in FIGS. 55-58 .

It must be kept in mind that, for the evaluation of the angulardistribution, it is often only the aspect ratio of the mask structurewhich is critical.

Thus the sizes of the openings in masking structures for thin masks onlyslightly thicker than the maximum projected range in the mask materialcan lie in the micrometer or submicrometer range.

Such monitoring structures are preferably arranged as arrays consistingof many individual structures, so that a global optical evaluation(i.e., over a surface of several mm² or cm²) can be conducted.

In contrast, for the same aspect ratios and, for example, maskthicknesses in the millimeter range, the opening sizes can lie in themillimeter or centimeter range. In these cases, it is also possiblewithout extraordinary technical effort to evaluate individual structureswhich are not arranged in an array.

Proposed mask structures:

1. fixed mask thickness, mask openings of various geometries→variationof the aspect ratio;

2. variable mask thickness, fixed geometry of the mask opening→variationof the aspect ratio;

3. combinations of 1 and 2;

4. through the arrangement of several arrays (or of individualstructures) each consisting of 1, 2, or 3 angles different from eachother, the directional dependence of the angular distribution can bemonitored.

Circular arrangements are also conceivable.

As shown in FIG. 58 , in addition to the concentric circular rings,individual circles and circular rings of various dimensions are alsoespecially advantageous for the monitoring of the angular distributionof the ions transmitted by the energy filter.

The core of the last aspects explained above consists in that the(essentially) dose-dependent modification of the (preferred) opticalparameters of a material is to be used for the “as-implanted” monitoringof the energy filter implantation process As a result, by means of anoptical measurement (for example), the resulting implantation obtainedcan be monitored as completely as possible with respect to its mostimportant parameters without the need to conduct any complicatedpost-processing (e.g., annealing and application of metallic contacts).

It therefore becomes possible to detect faulty implantations cheaply andquickly and possibly to sort out the affected wafers.

FIG. 59 illustrates a skillful adaptation of a profile transitionbetween two implantation profiles A and B, so that the resulting overallconcentration profile can, for example, produce the homogeneous profiledesired. This can (but does not necessarily have to) be advantageous inparticular for layer systems of two layers as shown in this figure.Proposal for a realization with the following sequence of processes:

(1) doping the lower layer (implant B);

(2) growing the upper layer;

(3) doping the upper layer. There are only limited possibilities forconfiguring the high-energy tail of implant A, but the low-energy tailof implant B can be influenced in particular by the introduction of asacrificial layer as described in “Point 15: Modification of the dopingprofile in the substrate by means of a sacrificial layer”.

Proposal for a realization with the following sequence of processes:

(1) growing a sacrificial layer;

(2) doping the lower layer (implant B);

(3) removing the sacrificial layer;

(4) growing the upper layer;

(5) doping the upper layer.

The concepts explained above make it possible to developproduction-worthy implantation methods for the semiconductor industry,i.e., an economic application of implantation methods in an industrialproduction process. In addition to the homogeneous dopings to berealized by simple triangular filter structures, the concepts explainedhere make it possible in particular to realize complex vertical dopingconcentration curves in a highly flexible manner (multifilter concept)with a low angular distribution of the implanted ions. In particular,all types of doping concentration curves can be approximated by the useof triangular filter structures in conjunction with collimatorstructures. Another important aspect pertains to the suppression ofartifacts, which falsify the ion current measurement on the substrate.

In conclusion, it should be pointed out once again that the measuresexplained above under Points 1-19 can be applied by themselves alone orin any desired combinations with each other. For example, the explained“end-of-life” detection can be applied to a filter mounted in a frame,but it can also be applied to filters mounted in some other way.

The above-explained wafer, furthermore, can be a semiconductor wafer,but it can also consist of some other material to be implanted such asPMMA.

REFERENCES

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What is claimed is:
 1. A method of doping a wafer, comprising:implanting ions into a wafer by irradiating the wafer with an ion beamusing an implantation device, the implantation device comprising afilter frame and a filter held by the filter frame, wherein the filteris irradiated by the ion beam passing through the filter to the wafer,and wherein the filter has an even base surface on one side andprotruding microstructures on an opposite side, and wherein the filteris arranged such that the protruding microstructures of the filter faceaway from the wafer and towards the ion beam, and wherein the filter istilted in such a way with respect to the wafer during the implantingthat the even base surface of the filter is arranged at an angle greaterthan zero with respect to a surface of the wafer which faces towards theion beam.